The invention relates generally to valve return spring arrangements for internal combustion piston engines, fluid pumps and similar machines.
The most commonly-used method for valve return springs in internal combustion piston engines today is the helical compression spring which is coupled to the valve at one end and is stationary at the other end, and is coaxial with the valve axis.
This method, while providing the basic function of springing the valve mechanism, is known to have undesirable characteristics that can compromise the valvetrain system function and ultimately the total engine function. One compromise that has been well-studied is the tendency for destructive coil surges that tends to increase as engine RPMs increase. The active coil of the helical-compression-type valve spring is often found to have too-low a natural frequency relative to the valve actuation frequency which is equal to the camshaft speed and is ½ of the engine operating speed for a 4-stroke engine. Coil surge is unwanted vibration of the active coil of a spring that causes cyclic increases and decreases in the forces that the spring is intended to produce and results in several problems: 1) higher fatigue stresses in the spring which requires the spring designer to adjust the nominal stresses downwards by using a heavier wire which tends to lower the coil frequency further and compounds the problem; 2) a coil surge away from the valve spring retainer during the valve lift event causes a lapse of force that may result in separation and violent re-engagement of the cam and follower that results in damage to the engine; 3) a coil surge away from the valve spring retainer at the end of a valve lift event causes a lapse in force that may result in valve bounce which can cause an undesirable exchange of air into or out of the cylinder that compromises engine performance; 4) a coil surge towards the valve spring retainer during the valve lift event may cause higher loading at the cam/follower interface and higher torsional loading for the camshaft timing drive system which exacerbates fatigue and wear of other engine components and necessitates the use of heavier more-expensive components. The problem of coil surge has been dealt with in several ways, including: 1) reducing the valve lift to reduce the spring force requirement which enables a higher frequency spring design, but may compromise engine performance; 2) using multiple concentric spring arrays to raise the coil frequency of the valve spring but increases the outside package diameter and requires use of a heavier valve spring retainer which increases the cost of the engine; 3) incorporating a spring damper to frictionally inhibit coil surge, or having concentric springs which interfere with each other to cause frictional damping—either of these may cause wear of the spring that may result in failure.
Another consideration for a valve spring may be the size and location of the spring package as it relates to other essential components in the engine cylinder head such as a spark plug, a direct-injection fuel injector or other features such as camshaft bearing structures and cylinder head bolt seats. The helical-compression-type valve spring, as it is applied almost universally, is concentric with the valve. Hence, the radial package around a valve axis in the upper part of a cylinder head is the outside radius of the valve spring which in some instances can lead to a compromise. In modern diesel engines, for example, it is often preferred to have the fuel injector in the center of a four-valve array with all four valves being parallel with the cylinder bore axis. However, for smaller cylinder bore diameters, due to the proximity of the helical-compression-type valve springs with the fuel injector, the four-valve array must be splayed outwards such that the valves are not parallel with the cylinder bore axis in order to obtain the necessary clearance between the fuel injector and the valve springs resulting in a compromised combustion chamber.